Developments in combustion engine technology have allowed for the use of gaseous fuels instead of diesel as a fuel, without sacrifices to vehicular performance or delivery. As used herein, gaseous fuels are defined as those fuels that are in the gas phase at standard temperature and pressure, which in the context of this application is 21 degrees Celsius (° C.) and 1 atmosphere (atm). Exemplary gaseous fuels include natural gas, methane, hydrogen and other combustible hydrocarbon derivatives. Because of its ready availability, low cost and potential for reducing particulate emissions, natural gas is increasingly used as the gaseous fuel of choice, to fuel a range of industrial and vehicle engines, including mine trucks, locomotives, ships and other heavy goods vehicles (HGVs). To increase the energy density of natural gas storage, especially on vehicles, liquefied natural gas (LNG) is an attractive solution compared to compressed natural gas (CNG) because of the higher energy density that can be achieved at much lower storage pressures.
Natural gas fuelled direct injection engines that can deliver similar performance profiles to diesel engines require the fuel to be delivered to the engine at high pressures, to overcome the in-cylinder pressure for late-cycle injection. In addition, a storage vessel for storing a cryogenic fluid, such as LNG must be adequately thermally insulated to maintain the stored LNG at cryogenic temperatures. Cryogenic temperatures are defined herein to be temperatures at which gaseous fuels will remain in liquefied form at a predetermined storage pressure.
Thermal insulation prevents heat transfer from the surrounding environment to the cryogen space within the storage vessel, because heat entering the cryogen space can cause the LNG to boil, increasing the vapor pressure within the vessel. If the vapor fluid pressure exceeds a predetermined pressure limit, to prevent any damage to the storage vessel, fuel pump or any other parts of a cryogenic tank assembly, a pressure relief valve is triggered to vent vapor from the cryogen space. It is undesirable to vent vapor from the cryogen space, so in addition to the thermal insulation, conventional cryogenic tank assemblies also avoid locating anything inside the storage vessel that can introduce heat into the cryogen space. The applicant has patented some inventive cryogenic tank assemblies that do place a pump inside the cryogen space with a hydraulic pump drive located outside the storage vessel, for example U.S. Pat. No. 7,293,418 B2. The temperature of hydraulic fluid employed by the hydraulic pump drive can be in the range of 65° C. and 90° C., and when compared to a typical temperature for LNG of on the order of −160° C. forms a very large temperature gradient therebetween. In addition to the challenge of preventing heat from entering the cryogen space, there are other challenges associated with locating a pump drive unit inside the storage vessel, such as preventing the freezing of hydraulic fluid lines if the drive unit is a hydraulic drive unit.
Accordingly, there is a need for an improved cryogenic fuel storage and pressurizing assembly suited for large-scale cryogenic storage vessels, that increases the fuel storage volume within the designated fuel storage space on board the vehicle, while mitigating the challenges associated with locating a hydraulic pump drive unit within the cryogenic fuel storage and pressurizing assembly.